Effects of Snow Cover on Soil Heat Flux and Freeze-thaw Processes

Snow cover insulates the soil surface from cold air in winter, thereby influencing the transfer of heat and water, and the freeze-thaw processes. To evaluate the effect of snow cover on land-atmosphere energy exchange, a field study was conducted in Hokkaido, Japan. The soil froze to a depth of 20 cm in early December while the snow cover was relatively thin. The soil then started thawing from the bottom of the frozen layer after the snow cover reached a thickness of 70 cm in January. The thawing was induced by a dramatic reduction in the heat flux from the soil surface to the snow, while the upward flux from the deep (>1 m) soil zone provided a steady supply of heat. The magnitude of heat flux in the deep soil zone is normally much smaller than the flux in the near-surface soil exposed to large fluctuations of air temperature. However, when the soil surface was insulated by snow, the magnitude of the near-surface heat flux decreased and became comparable to that of the flux in the deeper soil zone, indicating that the effects of the latter increased after the insulation of the soil surface. Since heat flux is sensitive to thermal conductivity, accurate estimation of the thermal conductivities of soil and snow is critical for a quantitative understanding of the freeze-thaw processes.